The technique described in Japanese Patent 2592340 (page 5, left column, FIG. 2, FIG. 6, etc.) is a known example of the technique relating to a legged mobile robot, particularly to the leg of a legged mobile robot. In this prior art, the configuration is such that an electric motor (80) for driving a knee joint is disposed at a thigh link (70), an electric motor (88) for driving an ankle joint is disposed at a shank link (86), and speed reducers (84)(92) disposed coaxially with the joints are driven by belts (82)(90), whereby the drive force necessary for walking is obtained.
When the legged mobile robot is moved, particularly when the robot is moved at a high speed, a strong inertial force is generated in the leg. Therefore, the weight of the leg, particularly on the distal end (specifically, the foot), is preferably light so that the inertial force generated in the leg during movement or locomotion is reduced. However, in the prior art described above, since the motor for driving the ankle joint is disposed at the shank link and the speed reducer for reducing the speed of the output thereof is disposed coaxially with the ankle joint, the distal end of the leg is heavy, which leaves room for improvement in terms of reducing the inertial force.